1. Field in the Industry
The present invention concerns a TiAl turbine rotor used as a part of turbochargers for internal combustion engines, and a method of manufacturing the same.
2. State of the Art
To date, turbine rotors for turbochargers of internal combustion engines have been manufactured by bonding a shaft made of a structural steel to a turbine wheel made by precision casting of, for example, Ni-based super alloy Inconel 713.degree. C. having good high temperature strength by friction bonding or electron beam bonding.
For the purpose of improving heat resistance of the turbochargers and to enhance responsibility of engines by lowering inertia caused by lowered weight of the turbine wheels, ceramics turbine rotors made of silicon nitride have been practiced.
The ceramics turbine rotors have drawbacks such as,
1) that the wheels must be thicker than those of conventional metal products due to lower rigidity of the material; and PA1 2) that balance of thermal expansion between the wheel and the casing is difficult to achieve due to lower thermal expansion of ceramics.
As a new material to replace ceramics, TiAl alloys drew attention because of their such low specific gravity as 3.8, which is of the same level as those of ceramics, a high specific strength (strength by density) at high temperature, which is equal to or higher than that of Inconel 713.degree. C., and a thermal expansion coefficient near those of metals. Thus, it was proposed to use the TiAl alloys as the material for turbine wheels (for example, Japanese Patent Disclosure No. 61-229901). The TiAl alloys practically used are those containing TiAl intermetallic compound as the main component, and the alloy compositions vary in a certain range. In the following description, however, the alloys are collectively referred to as "TiAl".
The TiAl turbine wheels are made by precision casting or isothermal forging and then, bonded to shafts made of a structural steel to form the turbine rotors. For the way of friction bonding, which has been practiced for bonding wheels of conventional Ni-based superalloy and shafts of a structural steel, cannot be applied to bonding TiAl wheels. This is because, if friction bonding is employed, transformation of the structural steel at the time of cooling from austenite to martensite causes volume expansion of the steel, which results in residual stress, and even though the TiAl have much higher rigidity which ceramics lack, ductility at room temperature is so low as about 1%, and thus, cracking of the TiAl wheels may occur. Further, reaction of Ti in TiAl and C in the structural steel occurs to form titanium carbide at the bonding interface, and therefore, strength at the interface decreases.
As the solutions of these problems, there has been proposed to practice vacuum brazing or friction bonding using an intermediate parts of austenitic material which does not suffer from martensitic transformation (for example, Japanese Patent Disclosure No. 02-133183).
The former solution, vacuum brazing mentioned above, must be carried out in high vacuum, which necessitates longer operation period inclusive of vacuum evacuation, and the costs will be higher. The latter solution, use of intermediate parts, requires two stages of bonding, for example, the first bonding between the intermediate part and the shaft and the second bonding between the intermediate part and the turbine wheel. Thus, the costs for manufacturing are also high. Further, control of the intermediate thickness after bonding is difficult.
The shaft part of the turbine rotor made by bonding is subjected to, for the purpose of refining, hardening and tempering, and then, the surface of the shaft around the bonded part to be supported by bearings is subjected to, for the purpose of improving wear resistance, hardening by high HF-heating or by laser heating. In case of vacuum brazing, if the austenitizing temperature is above the melting point of the brazing metal, then the brazing metal melts again at heating for hardening. This will result in oxidation of the brazing metal and decrease of strength at the bonded part. In some cases the product rotors may be even destroyed during handling.
Under the above circumstances, TiAl turbine rotors have not been practically manufactured. Main bar to practical manufacturing is high cost of production. The inventors tried brazing under high frequency induction heating (hereinafter abbreviated as "HF-heating"). The trial succeeded and it was ascertained that TiAl turbine wheels and steel shafts can be bonded with high bonding strength and that TiAl turbine rotors can be manufactured with reduced costs.
In practice of the above technology it was experienced that, if the interfaces to be bonded by brazing are plane, placing the axis of the TiAl wheel and the axis of the shaft in accordance is difficult, and that eccentric bonding often occurred. Also, due to higher thermal conductivity of TiAl, conduction of heat from the TiAl wheel, which is exposed to a high temperature during use, to the shaft is so high that the shaft goes to a high temperature and as the result, seizure of bearings may occur.